A numerical model of Q-switched 4-level intracavity frequency doubled laser is proposed based on rate equation, Q-switching and effective energy storage time theory. The calculated peak power, pulse width and average power with repetition rate is derived and compared with experimental results of a Q-switched 532 nm laser. The calculated results fit well with the experimental results during the repetition rate range from 1kHz to 100kHz which shows that the numerical model is valid.